In the production of corrugated thermoplastic tubing using apparatus of the type referred to above it is necessary to provide for the escape of gas, typically air, from the space between the tube and the wall of the mold tunnel, since the gas would otherwise form a cushion preventing the complete expansion of the tube. This is obviously so in the case where the tube is expanded by vacuum forming, and in vacuum forming apparatus it is common practice to provide vents through the mold blocks for this purpose, as described for example in U.S. Pat. No. 3,751,541, dated Apr. 7, 1973, to W. Hegler. It is also necessary to provide for the escape of entrapped gas in the case where the tube is expanded by internal gas pressure, and such provision is described for example in the U.S. Pat. No. 3,981,663, dated Sept. 21, 1976, to G. P. H. Lupke. However, in the production of corrugated tubing, as distinguished from smooth walled tubing, the exhaust of gas or removal of entrapped gas from the inside of the mold tunnel presents a special problem since the gas tends to become trapped in discrete pockets in the troughs of the corrugations of the mold blocks. To ensure accurate conformity of the tube to the wall of mold tunnel it is necessary to provide for the easy removal of all of this gas during the molding process. Furthermore, by reason of the fact that such apparatus necessarily includes a large number of accurately formed similar mold blocks, it is desirable that the mold blocks should not be structurally complicated and costly.
The present invention is primarily concerned with novel mold box structures by which these results are accomplished.